The present invention relates to a compact disk storage medium and a compact disk drive, and, more particularly, to disk storage medium and to apparatus for recording and reproducing data to and from a disk storage medium. The invention also relates to apparatus for recording and reproducing data at a constant angular velocity to and from a disk storage medium, and for adjusting the rotational velocity of the disk storage medium.
Currently, compact disks (CDs) are utilized for recording digital audio signals thereon by a known optical process. A 12 cm CD has a data storage capacity of approximately 640 megabytes and an 8 cm CD has a data storage capacity of approximately 200 megabytes. Hence, such CDs are useful for the storage of large quantities of digital audio data.
Compact disk read-only memories, known as CD-ROMs, store digital data, other than audio data, which are recorded in the same manner as audio data is recorded onto a CD. CD-ROMs generally are used in the 12 cm format, where spiral tracks of digital data are stored on the CD-ROM by a known optical process after the digital data is double-coded with a "cross interleave Reed-Solomon code" (CIRC) and modulated by an eight-to-fourteen (EFM) modulation process. Generally, each sector (i.e., information unit) of a CD-ROM stores one subcode block of data consisting of 98 frames. Each frame in an audio CD contains 8 bits of subcodes "P" to "W".
CD-ROMs are increasingly being used for the distribution of computer software because of their large storage capacity. Such uses include the storage of application software, statistical data (e.g., software encyclopedias), manuals, etc. In addition to their large storage capacity, CD-ROMs are well suited for mass reproduction because of their decreasing production costs and low error and failure rates.
The CD-ROM is an optical-type storage medium in which digital data is stored thereon with the disk being driven at a constant linear velocity (CLV), that is, the angular rotation of the disk varies in relation with the radial distance from the center of the disk of the location at which data is stored. When reproducing, a spindle motor in a CD-ROM drive rotates a 12 cm CD-ROM at the rotational (i.e., angular) velocity of 200 revolutions per minute (rpm) to reproduce data stored in the outermost tracks and rotates the disk at the rotational velocity of 535 rpm to reproduce data stored in the innermost tracks. However, because of the large inertia of the spindle motor, the normal read or access time for reproducing the stored data increases when the spindle motor changes the rotational speed of the disk.
CD-ROM drives do not determine the address of a sector until sector address data is reproduced from the subcode portion of the sector. When a specified sector is to be reproduced, the optical pick-unit of the CD-ROM drive accesses a pseudo-random portion of the disk which is likely to include the specified sector, and the accessed sector address is identified in the reproduced subcode. The drive then accesses another sector until the specified sector is reproduced. Since the optical pick-up unit in the drive reproduces pseudo-random locations during a seek operation, the rotational speed of the disk is constantly changing, thus reducing the access time. In conventional devices, the seek time to reproduce one-third of the storage locations of a disk is approximately 200 microseconds, and since the power dissipation is 2 to 3 watts when the rotational velocity changes, such CD-ROM drives are not well-suited for systems having low power requirements, e.g., notebook or other portable-type computers. Nonetheless, CD-ROMs are popular due to their large storage capacities, fast access time and portability with respect to the storage capacity of floppy diskettes and the access time of magnetic tapes. However, since CD-ROMs are read-only type memory devices, there is a need to have a read/write type storage device which has a large storage capacity, a fast access time, and which is portable, and although magnetic tapes have storage capacities from 10 megabytes to several tens of gigabytes, their access speed is relatively slow, and thus, they are used primarily as backup storage for the contents of hard disks.
A writable compact disk, known as a write once compact disk (CD-WO), has been developed which has a format similar to the CD-ROM but which can be written onto only once. In addition, since the CD-WO is a constant linear velocity (CLV) type storage medium, it has the same problems as those of the CD-ROM previously discussed.